Plasma flame generators and spray guns utilizing an electric arc and a flowing gas stream passed in contact with the arc are generally known and have been used successfully for commercial and experimental purposes. These devices generally consist of an electrode arrangement striking an arc therebetween, a nozzle and means for passing a stream of gas in contact with the arc and through the nozzle.
In plasma flame generators of the non-transfer type, the arc is struck between an electrode pair, one of which is in the form of a nozzle, and the gas stream is passed in contact with the arc and through the nozzle. U.S. Pat. No. 2,922,869 typifies the early designs for such plasma generators. In generators of the transferred arc type which are generally used as torches for cutting, welding, and the like, the arc generally extends from an electrode such as a rod electrode to the workpiece through a nozzle, while a gas stream is passed concurrently through the nozzle with the arc. Plasma flame spray guns, in principle, merely constitute plasma flame generators in which means are provided for passing a heat fusible material into contact with the plasma stream where it can be melted or at least softened and propelled onto a surface to be coated.
A variety of plasma spray gun configurations have been devised for spraying into confined areas. These have generally been designed to the problems of coating inside diameters of holes. They virtually all have limitations for minimum size hole associated with physical sizes of electrodes and the channeling of plasma-forming gas, coolant and powder feed, as well as required minimum spray distance.
For example U.S. Pat. No. 4,661,682 (Gruner et al) describes a plasma spray gun incorporated sideways on the end of an elongated arm. Size of confined area spraying, e.g. minimum diameter of the hole being coated, is limited by the necessary combined lengths of the cathode and anode structures. U.S. Pat. No. 3,740,522 (Muehlberger) discloses an elongated plasma gun with an angular nozzle anode used in conjunction with a cathode for deflecting a plasma stream from longitudinal to transverse to the initial main axis of the gun. This apparatus is similarly limited in minimum size by the configurations of the components, coolant channeling out and back, and powder conduits. U.S. Pat. No. 4,596,918 (Ponghis) discloses an elongated anode with concentric channeling for coolant on a plasma torch, but does not teach means for deflecting the spray stream or injecting powder. Various configurations for powder feeding are illustrated in U.S. Pat. Nos. 4,696,855 (Pettit et al) and 4,681,772 (Rairden).
Therefore the practicality of plasma spraying into confined regions remains elusive. A particular type of confined region of extensive interest is illustrated by the slotted regions for mounting blades and vanes on hubs in gas turbine engines. Such areas are subject to extensive fretting wear from vibrations and other stresses in the assemblies during engine operation. Plasma sprayed coatings have been developed which minimize such wear and can be used for repair of the components. These coatings have been used in the mounting slots but only where the slots are large or designed without overhangs so that a plasma spray stream can be directed from outside a slot onto the internal surfaces. However it is desirable to utilize dovetail slots to better retain the blades and vanes. Small dovetail slots are being designed into newer gas turbine engines. Heretofore small-type dovetail slots could not fully be coated. Also it is important for a coating to be sprayed nearly perpendicularly to the surface. Spraying from outside a slot onto side walls does not achieve this goal and results in inferior coatings.